Pentateuch Envy, Anyone?

Steins? Their cups runneth over - way over, as this at-times very funny film will have viewers not so much beaming from the bimah as bemoaning where all the flour has gone?

Why, to build a Bar Mitzvah cake that would sink a Titanic - if that weren't already in play as the theme for the young Stein's Bar Mitzvah.

The only thing the Steins didn't come up with is getting Celine Dion to convert to Judaism. But, then, their's is the Bar Mitzvah that will go on … and on.

On the mark is Scott, whose "Keeping Up With the Steins" is a keeper. Indeed, such extravagant Bar Mitzvahs as depicted here once kept him up nights as a kid.

After all, why couldn't he, part of a famous family fiefdom of funny, have his own Bar Mitzvah to equal those of his buds?

Well, for one, he wasn't Jewish …

But he does have the wry humor that is the bread and butter - okay, margarine - of Jewish sensibility that makes for some very comical scenes in the film, opening this Friday.

Guilty as charged - if not Jewish guilt as charged. "In a way, this is the Bar Mitzvah gift I never got," he kibitzes of "Keeping Up With the Steins."

"I always wanted one. I went to a lot of Bar Mitzvahs as a kid, every weekend."

And, often as a player, although not halachically speaking. "I played in a lot of the bands at the Bar Mitzvahs, too."

What could be cooler - besides the ice sculpture - than a Bar Mitzvah where everyone's having a "Hava Nagilah" of a time? "I'd hang out with the girls, I'd dance with them."

But, alas, he'd go home yarmulke in hand. For when all was said and done - and chanted - he wasn't Jewish.

"And I never knew - it was never clear-cut - when I became a man."

Man-to-man, nothing to worry about, Scott. ("It used to be Scottie; now it's Scott," his Dad Garry kiddingly complains.)

"I tried to have a big party of my own when I was 13. And it was great, on the set of 'Mork & Mindy,' " which his famous father directed and produced.

But nanu-nanu … "No one under 18 was allowed on the set."

Which means that the only one wired for laughs - without braces - was Robin Williams.

Robbed? Nah, says Marshall, who got to stage three Bar Mitzvahs - no treif-fecta here - in his first feature film. "And I got to learn the Haftorah," with special tutorials from the talented Cantor Chaim Frenkel, who served as technical adviser on "all things Jewish," ensuring, according to Mark Zakarin, the "Steins" writer-producer, that "all the Bars were crossed and all the Mitzvahs were dotted."

"All involved were keeping me kosher," says Marshall.

Heck, he got their hechsher, but that still didn't necessarily mean it would all come together. Indeed, like a tight kipah stuck on with a bobbie-pin, not everyone could pull it off, including the director his first time out. "I tried to write a Bar Mitzvah comedy about 10 years ago, but I couldn't finish it. It just didn't have the heart."

At the heart of home and hearth these days is a Jewish support group: "Hi! My name is Scott Marshall, and I'm surrounded by Jews"? Maybe. "My wife is Jewish, and now I have a bubbie," he says lovingly.

"When did I become a mesch? At my wedding, when my father-in-law made a speech that made me know that now I am a mensch."

But were the wise-ass scenes in the film a menschedick thing to do to his own father? Garry Marshall in the nude? Playgirl is so not calling.

"It was great to have family around," he says of star Garry. "They know their lines and show up on time," he jokes.

And if those lines show up on the star's tuchas, too?

So, what's the skinny on getting his father to skinny-dip?

"The producer asked him," he says. "I wasn't about to ask him."

But Father's Day comes every day: "He had no problem," says Scott of Garry taking a dive. "Especially when we said he'd be doing the scenes with Daryl Hannah."

If that made a splash with Dad, son Scott had already done that as a director. A graduate of Northwestern University and the American Film Institute, where he earned a master's in directing, "I had spent a lot of times on sets" as second-unit director for many of his father's films.

Runaway bruchas? "It's not really strange to work with him. We've always had a friendly relationship."

The father-son storm at the eye of the film is not the Marshall oys; but son Scott does recognize some of the snarly subplots. "I was able to relate to the pushy best friend who gets you into trouble, and the shyness meeting girls."

Starring in Chavez the rock band served as a hot wax Hillel of sorts later on; mixing tracks was the ultimate mixer. Indeed, one of Chavez' numbers is in the film and Scott still numbers himself a band member.

What's next - a klezmer recording of "Marshall Does Mitzvot"?

"No," he says laughing, assuring he is not guilty of Pentateuch envy.

But Scott does relish the sound of laughter, and he finds most of that coming from "coming of age" movies. "I have such a nostalgia for that," says Marshall. "All the John Hughes movies … I loved them."

His breakfast club these days is made up of the likes of Daryl Hannah, Jami Gertz, Jeremy Piven - in the pivotal part of father of the Bar Mitzvah kid, portrayed by Daryl Sabara - starring in "Steins."

If these aren't the "Flamingo Kid" - "That's my favorite coming-of-age movie," says Scott of his dad's very funny and famous film - they can at least stand on their own two feet.

And so does Scott Marshall. Is it possible he may be Hollywood's new "IT" boy?

Boy? Did I say boy?

"Hey," says the director of the ultimate professional Bar Mitzvah home movie, "I'm a man now!"